Remove ASSERTion about increasing TcLevels

This removes an ASSERTion that TcLevels should increase by
exactly one in every implication. While this is a sensible
goal, it's not true today, and we should not be crippling
DEBUG for everyone while debugging this.

The ASSERT was added in 261dd83c

compiler/typecheck/TcSimplify.hs

@@ -1495,7 +1495,8 @@ solveImplication imp@(Implic { ic_tclvl  = tclvl
   = do { inerts <- getTcSInerts
        ; traceTcS "solveImplication {" (ppr imp $$ text "Inerts" <+> ppr inerts)
 
-       ; when debugIsOn check_tc_level
+       -- commented out; see `where` clause below
+       -- ; when debugIsOn check_tc_level
 
          -- Solve the nested constraints
        ; (no_given_eqs, given_insols, residual_wanted)
@@ -1544,10 +1545,15 @@ solveImplication imp@(Implic { ic_tclvl  = tclvl
     -- TcLevels must be strictly increasing (see (ImplicInv) in
     -- Note [TcLevel and untouchable type variables] in TcType),
     -- and in fact I thinkthey should always increase one level at a time.
+
+    -- Though sensible, this check causes lots of testsuite failures. It is
+    -- remaining commented out for now.
+    {-
     check_tc_level = do { cur_lvl <- TcS.getTcLevel
                         ; MASSERT2( tclvl == pushTcLevel cur_lvl
                                   , text "Cur lvl =" <+> ppr cur_lvl $$
                                     text "Imp lvl =" <+> ppr tclvl ) }
+    -}
 
 ----------------------
 setImplicationStatus :: Implication -> TcS (Maybe Implication)